The invention relates to an electrodeless metal vapor discharge lamp comprising a lamp vessel which is sealed in a vacuum-tight manner and is further filled with a rare gas, this lamp being provided with a core of magnetic material in which a high-frequency magnetic field can be induced by means of an electric supply unit and a winding arranged to surround the core, an electric field being produced in the lamp vessel during lamp operation. Such a lamp is known from U.S. Pat. No. 3,521,120.
This U.S. Patent Specification discloses an electrodeless fluorescent low-pressure mercury vapor discharge lamp having such a form that it is suitable to serve as an alternative to an incandescent lamp intended for general illumination purposes, for example for use in a private house. An electrodeless mercury vapor discharge lamp has a favorable efficiency as compared with an incandescent lamp, while with a suitable choice of the luminescent material on the inner wall of the lamp vessel a satisfactory color rendition can be obtained. The lamp is operated at a high-frequency supply voltage, that is to say that the supply voltage has a frequency which is generally higher than 20 kHz.
A problem which arises during operation of the above lamps is that the electromagnetic field produced in the lamp extends outside the lamp vessel in the neighbourhood of the lamp. As a result, especially due to the magnetic component of the field, annoying and disturbing signals occur, for example, in electrical apparatus, such as radio receivers and the like, arranged in the proximity of the lamp. For the intensity of the electromagnetic field outside the lamp international standards have to be observed, which can easily be exceeded without the use of special measures. Special measures for suppressing the said undesired phenomena which are taken in the lamp are, for example, the application of a thin transparent conductive layer to the inner wall of the lamp vessel, as described, for example, in U.S. Pat. No. 4,171,503, or the provision of a metal anti-interference ring around the lamp vessel (see U.S. Pat. No. 4,187,447). However, it has been found that despite these measures the disturbing field is insufficiently suppressed and also that the heat developed in the core of magnetic material is liable to rise during operation to such a value that the efficiency of the lamp is adversely affected.